1. Field of the Invention
The present invention relates to an image processing apparatus and its control method. More particularly, the present invention relates to enabling reliable decoding of the encoded information embedded in a source document when carrying out enlarging or reducing scanning of the source document in which the encoded information such as security additional information is embedded.
2. Description of Related Art
Recently, as office IT has been promoted, there has been a growing interest in security. For example, cases of leaking customer information corporations retain have occurred frequently, which threatens top secrets of organization or personal privacy and becomes an issue of public concern.
To deal with such problems, an IT (Information Technology) mechanism has been introduced which provides access rights to electronized top secret information or provides a firewall with a monitoring device so as to prevent leakage to the outside of the corporations. Alternatively, a measure is taken to forbid bringing a transportable medium such as a note PC and USB memory in or out of an office. As for the electronized top secret information, the measure using the IT mechanism as described above can prevent the leakage to some extent. This is because the electronic information cannot be accessed without using IT, and hence it is easy to introduce the mechanism thereinto.
On the other hand, if the top secret information of organization or personal privacy information is printed on a paper medium with an image forming device, then trying to maintain the security by checking or forbidding taking out the medium will involve circumventing the conventional IT mechanism. Thus, it is harder to forbid taking out the paper medium than to limit taking out the electronized top secret information, which makes it difficult to maintain the security.
To deal with the problem, various countermeasures against information leakage have been taken conventionally.
As a first countermeasure against information leakage, systems have been developed which embed tracking information in a paper medium itself in addition to a main image (source document image) at the printing using information embedding technology such as a known digital watermarking technique and two-dimensional bar code. According to these systems, even if the information leakage of the paper medium occurs, the tracking information can be extracted by analyzing the leaked source document. Thus, as for unexpected taking out of the source document, it is possible to find out who is to blame from the tracking information, and hence deterrent effect can be expected.
As a second conventional countermeasure against information leakage, systems have been developed which embed copy restricting information for forbidding copy on a paper medium itself when carrying out printing using known digital watermarking technique or two-dimensional bar code technology. In the systems, a corresponding image forming device detects whether copy is permitted or not by extracting the copy restricting information at the copying, and is able to control carrying on or halting the copy page by page. In addition, systems are also proposed which permit only specified users to make copies by embedding not only the copy restricting information simply indicating pros and cons of the copying, but also password information or authorized user information as condition information. Such a system is disclosed in Japanese Patent Laid-Open No. 2003-280469. The foregoing tracking information and copy restricting information are generically referred to as “security additional information” from now on.
When performing a scaling copying conventionally, regardless of whether the security additional information is embedded in a source document or not, different scaling processing is performed in the paper feed direction (subscanning direction) of the source document and in the direction perpendicular to the paper feed direction (main scanning direction). More specifically, as for the scaling in the paper feed direction (subscanning direction) of the source document, it is the so-called optical scaling that achieves the scaling by changing scanning speed. In contrast, as for the scaling in the direction perpendicular to the paper feed direction (main scanning direction), it is carried out by combining digital image data acquired by scanning with digital scaling processing. Applying the optical scaling to the scaling in the subscanning direction in this manner makes it possible to reduce image quality degradation due to scaling as compared with the case of applying the digital scaling to the scaling in the subscanning direction.
On the other hand, to extract the security additional information embedded in the source document at a time of scaling copying, it is desirable to use (full-scale) digital image data without being scaled up or down as the image data for extraction. For this reason, processing of returning the image, which is scaled up or down in the subscanning direction by the optical scaling, to its full-scale (referred to as “reverse scaling processing” also) is carried out separately for extracting the security additional information.
However, if arbitrary magnification scaling is allowed in the optical scaling in the foregoing conventional technology, there is a danger of eliminating the security additional information embedded in the source document because of a calculation error involved in the reverse scaling processing for returning to the full-scale data.
Considering preventing deterioration in the image quality when carrying out the foregoing scaling, the optical scaling is preferable because it does not perform thinning out or interpolation as the digital scaling does. However, when the optical scaling is performed on a source document in which a low-visible bar code is embedded as the security additional information, it will change in accordance with a scaling factor the grid spacing in the subscanning direction in the image data acquired by performing the optical scaling. In addition, there is a possibility of eliminating dots depending on the scaling factor.
As described above, the optical scaling is more effective when scaling an ordinary source document up or down because it can reduce deterioration in image quality. However, when scaling up or down a source document in which encoded information such as security additional information is embedded, there is a possibility of being unable to make analysis or of reducing the accuracy of the analysis when reading out the encoded information from a printed matter of the source document.